Glass ceramics having a low average thermal expansion or low average CTE (“coefficient of thermal expansion”) are already known in the prior art.
The average CTE is always specified for a particular temperature interval, this temperature interval being selected around the working temperature of the glass ceramic. Only if the average CTE in such a temperature interval around the working temperature is low enough can the intended dimensional stability be ensured when heating in the working temperature range.
Glass ceramics which are intended for use as a cooktop plate or stove window therefore have their composition adjusted to low expansion in the range of up to about 700° C. For example, the commercial glass ceramics for stove windows, for example Keralite® (described in EP 437 228) (Corning) or Robax® (SCHOTT) have average coefficients of thermal expansion of 0±0.3×10−6/K in the range of from 20° C. to 700° C. For a temperature range of from 0 to 50°, however, the values of the average CTE are only about −0.57×10−6/K (Robax®) and about −0.4×10−6/K (Keralite®).
Among the glass ceramics with particularly low thermal expansion, lithium aluminium silicate (LAS) glass ceramics are known as so-called “zero expansion materials”.
DE 1 596 860 and DE 1 902 432 relate to transparent glass ceramics of the Li2O—Al2O3—SiO2 system, and in particular Zerodur®, as well as to shaped glass-ceramic articles made of them. These glass ceramics do not contain any CaO.
The glass ceramics described in U.S. Pat. No. 4,851,372 (Lindig and Pannhorst) contain at least 1% by weight of BaO and also comprise Zerodur®M.
The glass ceramics mentioned in U.S. Pat. No. 5,591,682 (Goto et al.) comprise the commercial glass ceramic Clearceram®Z and contain relatively high BaO contents, which can have a negative effect on the processability of the glass ceramic.
Zerodur® is available as a commercial product in the following three expansion classes:
expansion class 2 CTE(0;+50° C.) 0±0.10×10−6/K
expansion class 1 CTE(0;+50° C.) 0±0.05×10−6/K
expansion class 0 CTE(0;+50° C.) 0±0.02×10−6/K,
the selected temperature range of from 0 to 50° C. comprising, for example, the working temperature range of mirror supports.
Modern requirements for different applications and therefore different working temperatures usually stipulate even more specific instructions relating to the thermal expansion behaviour.
An example which may be mentioned from EUV lithography is the expansion specification for mask substrates according to the specification SEMI Standard P37: the average CTE should not exceed 5 ppb/K, i.e. 0.005×10−6/K, in the range of from 19 to 25° C.
Substrates based on glass ceramic, as are described in EP 1 321 440, have been developed for such stringent specifications. This document describes how, particularly in the case of Zerodur®, the average CTE can be set even more accurately by controlled re- or post-ceramizing of the glass ceramic so that the aforementioned specifications for mask substrates can be achieved. It also describes that it is preferable and possible to shift the zero crossing of the CTE-T curve into the application temperature range, for example to a value in the range of from 15 to 35° C., and/or to set a particularly small gradient of the CTE-T curve at the zero crossing, for example less than 5 ppb/K2.
It has now been found that although Zerodur® seems particularly suitable as a substrate for EUV lithography owing to the good adjustability of the CTE, this glass ceramic has certain disadvantages in respect of the requisite processability of the substrates.
Mirrors for EUV lithography comprise a substrate covered with a multiple layer system, or multilayer, which makes it possible to produce mirrors with high reflectivity in X-ray range. A necessary prerequisite for achieving a high reflectivity is sufficiently low layer and substrate roughnesses in the mid and high spatial frequency roughness ranges (MSFR and HSFR).
For the definition of fine surface figure error, MSFR and HSFR, reference is made to the publication “Mirror substrates for EUV lithography: progress in metrology and optical fabrication technology” U. Dinger, F. Eisert, H. Lasser, M. Mayer, A. Seifert, G. Seitz, S. Stacklies, F. J. Stiegel, M. Weiser, in Proc. SPIE Vol. 4146, 2000. The HSFR comprises wavelengths of from 10 nm to 1 μm, and the MSFR comprises wavelengths of from 1 μm to 1 mm.
Defects on the substrate surface in the HSFR range lead to light losses by scattering out of the image field of the optics, or perturbation in the superposition of the wavetrain components. The HSFR is usually measured by atomic force microscopes (AFM) which have the necessary resolution.
A sufficient value of the surface roughness in the HSFR range for EUV lithography, for example 0.1 nm rms, can be achieved by conventional superpolishing methods even for glass ceramics such as Zerodur®. Since these methods generally deteriorate the fine surface figure error and/or the MSFR at least on aspheres, that is to say they produce defects in the low spatial frequency range and in the long-wave MSFR range, the superpolishing process generally has to be followed by a fine correction process. To this end, for example, the fine surface figure error and the long-wave MSFR components are brought into specification by beam processing methods, for example ion beam figuring (IBF). The advantage of this method is that such tools can fit accurately to the shape of, in particular, the typically aspherical surfaces. Such processing methods are based on sputtering processes. The sputtering rates depend on the chemical and physical bonding conditions in the solid body to be processed.
In commercial glass ceramics, the extra energy introduced by the beam processing method into the solid body to be processed can impair the surface roughness in the HSFR range. With Zerodur®, for example, a deterioration in the HSFR range of from 0.1 nm rms after superpolishing to 0.4 nm rms after IBF can be observed.
This problem is addressed in WO 03/016233 by proposing glass ceramics which contain microcrystallites with a small average size, i.e. substantially less than 50 nm, as substrate materials of X-ray optical components for EUV lithography. According to this document, the requisite roughness in the high spatial frequency range HSFR can thus be achieved after IBF.
With novel glass ceramics, there is therefore a need that they should not only have a low thermal expansion but also that this should be adjustable in a controlled way for different applications, and good processability should be achievable at the same time with a view to even better surface quality in respect of both fine surface figure error, MSFR and in particular HSFR. It is therefore an object of the present invention to provide a novel glass ceramic and optical elements or precision components made of it, with which the aforementioned problems can be overcome. In particular, such optical elements and precision components should have a very low surface roughness even after final processing steps such as IBF. It should preferably also be possible to adjust the zero crossing of the CTE-T curve, and the gradient of the CTE-T curve should also be as small as possible.